1. Field of the invention
The present invention relates to an automatic setting method for ATM network at office site, especially to an automatic setting method used in an xDSL device for setting virtual path identifier (VPI)/virtual channel identifier (VCI) and encapsulation capacity at ISP office site.
2. Description of the Prior Art
The rapid progress of computer network expedites the development of various network protocols. Among them, the asynchronous transfer mode (ATM) protocol in ADSL network is very attractive.
In conventional network, the transmission medium is shared by all communication terminals connected to the network and the data is sent through the transmission medium. Each of the communication terminals should be able to check whether the network is idle. In other word, the communication terminal must hold transmitted data when other communication terminal is transmitting data and then send data when the network is idle. The ATM protocol is developed to achieve point to point connection by switch, and the terminals in source end and destination end can use whole bandwidth of the transmission medium at the same time.
The data in current computer network is transmitted in packet form with different sizes. The packets with different sizes may also have different time delay. The time delay is difficult to estimate as network becomes more and more complicated.
The time delay problem becomes more serious when more and more video files and graphic files are transmitted through network. The time delay impacts the network as interference noise. Moreover, the variable-sized packet incurs difficulty to transmission of graphic and voice data.
ATM network overcome above-mentioned drawback by adopting small-sized and fixed-length packet for data transmission, this small packet is referred as cell. ATM network uses cell to enhance transmission rate, and reduce time delay and delay variation.
The conventional network implements the data link layer with hardware address. The hardware address is exclusive to an independent machine and there is no simple algorithm to resolve destination address by network system. Therefore, the source end just sends data to routing devices such as router and the delivery path of data is determined by network layer in OSI architecture.
However, the routing processed in the network layer will cause much computation effort. Therefore, the router will be the bottleneck of network transmission if the processor thereof is not such powerful.
Moreover, the router will examine each packet to determine its delivery path. After the source end and the destination end have already established connection, the router still needs to examine each packet between the source end and the destination end, even though the all the packets are transmitted through the same path between the source end and the destination end.
To address this issue, the ATM network solves routing problem by virtual connection, wherein the packets (cells) through a fixed path will not be examined again. In virtual connection, the path between two terminal ends is established by the first packet and then the data transmission between the two ends is directly managed by switch.
However, the ATM network is not completed complied with LAN infrastructure. For example, certain important fields are very important in cell frame of ATM such as Virtual Path Identifier (VPI), Virtual Channel Identifier (VCI), Payload Type (PT) and Cell Loss Priority (CLP) etc. The ATM switch used VPI/VCI field to guide the cell to desired output port.
The delivery path of the cell can be easily determined by VPI/VCI field because the VPI/VCI field has small bit size. The cells with the same VCI will be delivered through a virtual channel, and the cells with the same VPI will be delivered through a virtual path.
However, the ATM network is not completed complied with LAN infrastructure, the ISP offices in different locations or different countries may have different cell format. For example, the VPI/VCI field format in Taipei ISP office may be different to the VPI/VCI field format in Beijing ISP office. Moreover, the encapsulation capacity in Taipei ISP office may be different to the encapsulation capacity in Beijing ISP office. Therefore, the ATM switch should be setup by ISP administrator. The modem of xDSL subscriber can only be used for switches associated with the same ISP office. For example, the xDSL modem for switch in DC ISP office cannot be directly moved for switch in New York ISP office.